1. Field of the Art
The present invention relates to a circuit for actuating print wires in an impact dot-matrix printer.
2. Related Art Statement
In the art of impact type dot-matrix printers using a row of stiff print wires, attempts have been made to increase the printing speed.
A known wire drive circuit is illustrated in FIG. 6, wherein solenoids L1-L4 for actuating individual print wires are connected at their one end to respective solenoid-selecting transistors Tr1-Tr4, and at their other end to a common power supply E through a power-on/off transistor Tr5. The solenoids L1-L4 are selectively energized by turning on the respective one of the solenoid-selecting transistors Tr1-Tr4 while the power-on/off transistor Tr5 is held on, whereby the corresponding print wires are selectively actuated to make dots on a recording medium. For example, signals SG1 and SG5 are applied to the solenoid-selecting transistor Tr1 and the power-on/off transistor Tr5, respectively, for a time span T1, as indicated in FIG. 7, to turn on these two transistors Tr1 and Tr5, whereby the solenoid L1 is energized with an electric current I1 flowing therethrough. Thus, the corresponding print wire is actuated. When the two transistors Tr1 and Tr5 are turned off after the time span T1, electric energy produced by the solenoid L1 is discharged through a discharge circuit which includes a diode D1, the power supply E, a diode D5 and the solenoid L1.
In the case where the signal SG1 is applied again to the transistor Tr1 after the discharge current Ia flowing through the above-indicated discharge circuit, namely through the solenoid L1, has been reduced to zero, there is no problem. In this connection, it is noted that the minimum operating interval of any print wire is limited primarily by mechanical factors, and therefore the minimum interval of energization of the corresponding solenoid is also limited. Due to this minimum time interval limitation, there is always a sufficient time between the successive energizations of the solenoid L1, for example, during which the aforementioned discharge current Ia may be reduced to zero. Further, for improving the printing quality, there is a comparatively less need to actuate the same print wire at reduced intervals. Therefore, the problem of the discharge current Ia does not exist when the same solenoid is energized repeatedly.
However, there arises a problem when different print wires are actuated one after another. Stated more specifically, to increase the operating frequency of the print wire, the period at which the signals SG1-SG4 are generated should be shortened. If, for instance, the signals SG5 and SG2 are generated to turn on the transistors Tr5 and Tr2 for energizing the solenoid L2 while the energy produced by the previously energized solenoid L1 is still being discharged as the current Ia (FIG. 7), the above-indicated discharge circuit including the solenoid L1 is shorted by a circuit which is constituted by the diode D1, power-on/off transistor Tr5 and solenoid L1. Consequently, the impedance of the discharge circuit is reduced, and the attenuation of the discharge current Ia may not be completed in a short time. If the solenoid L1 is energized again in this condition, the current I1 to energize the solenoid L1 is added to the remaining discharge current Ia. Repetition of the above cycle will cause a progressive increase in the current I1 which energizes the solenoid L1, and eventually result in the solenoid L1 being kept energized. For this reason, a comparatively long time is required between the generation of the signal SG1 and the generation of the signal SG2, if the signal SG2 following the signal SG1 is followed by the signal SG1 as in the above case. Thus, it is impossible to start energizing the solenoid L2 soon after the signal SG1 has been removed.
Hence, in the above-discussed arrangement of the known wire drive circuit, it is difficult to reduce the interval of generation of the signals for energizing the solenoid for a print wire and the solenoids for the other print wires, in a repeated fashion. Namely, it is difficult to increase the operating frequency of the print wires.
For solving the above problem, the drive circuit may be modified as indicated in FIG. 8. In this modified circuit, plural power-on/off transistors Tr5-Tr8 are provided correponding to the individual solenoids L1-L4, so that the solenoids L1-L4 are energized when the respective solenoid-selecting transistors and the corresponding power-on/off transistors are turned on. This arrangement permits the attenuation of the aforementioned discharge current in a shorter period. Therefore, the solenoids may be energized repeatedly at an increased frequency.
However, the modified drive circuit of FIG. 8 uses a comparatively larger number of components (including diodes D6-D8 and transistors Tr6-Tr8), which inherently pushes up the cost of the drive circuit.